Circulating cholesterol is carried by plasma lipoproteins--particles of complex lipid and protein composition that transport lipids in the blood. Low density lipoproteins (LDL), and high density lipoproteins (HDL) are the major cholesterol carriers. LDL are believed to be responsible for the delivery of cholesterol from the liver (where it is synthesized or obtained from dietary sources) to extrahepatic tissues in the body. The term "reverse cholesterol transport" describes the transport of cholesterol from extrahepatic tissues to the liver where it is catabolized and eliminated. It is believed that plasma HDL particles play a major role in the reverse transport process, acting as scavengers of tissue cholesterol.
The evidence linking elevated serum cholesterol to coronary heart disease is overwhelming. For example, atherosclerosis is a slowly progressive disease characterized by the accumulation of cholesterol within the arterial wall. Compelling evidence supports the concept that lipids deposited in atherosclerotic lesions are derived primarily from plasma LDL; thus, LDLs have popularly become known as the "bad" cholesterol. In contrast, HDL serum levels correlate inversely with coronary heart disease--indeed, high serum levels of HDL are regarded as a negative risk factor. It is hypothesized that high levels of plasma HDL are not only protective against coronary artery disease, but may actually induce regression of atherosclerotic plaques (e.g., see Badimon et al., 1992, Circulation 86 (Suppl. III):86-94). Thus, HDL have popularly become known as the "good" cholesterol.